Transmission systems for high-speed track-laying vehicles or vehicles with non-steerable wheels

ABSTRACT

A transmission for high-speed track-laying vehicles or vehicles with non-steerable wheels having an engine adapted to produce substantially constant power over a wide range of speeds comprises a gearbox and a hydrodynamic torque converter. The transmission further comprises reversing gearing and epicyclic output gears. A positive clutch couples the hydrodynamic torque converter to the gearbox output shaft. Two freewheel clutches are respectively driven by the vehicle engine and by the hydrodynamic torque converter. By this means the highest drive speed is selected at all times. The transmission further incorporates steering means adapted to procure continuous variation of the turning radius. The transmission is particularly suitable for tanks.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention concerns transmission systems for high-speedtrack-laying vehicles or vehicles with non-steerable wheels.

2. Description of the Prior Art

High-speed track-laying vehicles, generally intended for military uses,must as far as possible be capable of moving as fast as possible onroads, of crossing the most severe obstacles and of retaining under allcircumstances the greatest possible mobility or "agility".

These vehicles must be capable of achieving speeds well in excess of 60kph and even as high as 80 kph, which distinguishes them from othertrack-laying vehicles such as those used in public works, which as ageneral rule do not have to move as fast.

In both cases steering the vehicle is based on the general principlewhereby the relative speed of the tracks is varied, but it will bereadily understood that the transmission systems used are not designedin the same manner, given the much more severe conditions that prevailin the case of steering high-speed track-laying vehicles because of thehigh power that passes to the tracks. In the case of earthmoving plant,this power is always relatively modest.

What is more, accurate control at high speed requires a steering systemprocuring continuous variation of the turning radius.

For these reasons there is an increasing tendency to use for steeringmodern vehicles, and in particular tanks, hydrostatic transmissionsystems, in particular transmissions incorporating a variable deliverypump (with axial pistons and inclinable plate, or with inclinable ornon-inclinable barrel) and a fixed capacity hydraulic motor. Fittingthese is facilitated by the fact that the same oil can be used for thesteering transmission and the gearbox, and this oil can be the same asthat used for the diesel engine.

Thus there are known transmission systems for high-speed track-layingvehicles or vehicles with non-steerable wheels comprising a gearboxequipped with a hydrodynamic torque converter functioning in a powersplitting arrangement, steering means featuring continuous variation ofthe turning radius and reversing gearing associated with epicyclicoutput gears.

Various techniques have already been put forward for improving theperformance of known transmissions and reference may usefully be had toFrench patent applications Nos 2 450 189 and 2 602 481 from the sameinventor as the present application and the contents of which are herebyincorporated by way of reference.

In French patent application No 2 450 189 it is proposed to use on thesteering shaft a variable speed drive the primary of which is driveneither from the primary of the torque converter of the transmission orfrom the secondary of this converter, according to which of these twomembers is rotating faster, so conferring on the vehicle the minimalturning radius.

In French patent application No 2 602 481 it is further proposed toprovide a transmission with a single-output steering device: in thiscase the output of the gearbox is connected to the annulus gear of afirst epicyclic gear (with fixed sun gear and planet carrier connectedto the rotating part of the braking device) and the steering devicecomprises a rotary member coupled to the sun gear of a second epicyclicgear with the same ratio (with the planet carrier fastened to that ofthe first gear and an annulus gear connected to the motion output shaftwhich extends to the braking and steering devices).

The state of the art is further illustrated by French patents Nos. 2 606713, 2 540 058, 2 468 790, 2 395 873 and 2 377 923, filed by thisapplicant, and British patents Nos. 1 456 717 and 2 006 899.

In parallel with this work there is considerable research on driveengines, especially on supercharged diesel engines.

In particular, researchers are attempting to develop turbines withseparate combustion chambers driving a compressor ("hyperbar" system):as yet the cost of these solutions would seem to be prohibitive,however. A more promising line of research is into using two-strokediesel engines: in particular it seems possible, by appropriatelyadapting the exhaust valves, to achieve a high degree of superchargingas a result of a higher pressure of the exhaust gas driving thesupercharger turbine, even at low engine speeds, which results inconstant power from speeds in the order of half the maximum speed.

An object of the invention is to provide a transmission that isparticularly well suited to engines supplying constant power over a widespeed range, for example in a ratio of 1:2.

Another object of the invention is to provide a transmission of simpleand rugged design enabling the number of gear ratios to be reduced whileimproving the performance of the vehicle using it.

A further object of the invention is to provide a transmission enablingthree modes of operation to be established, apart from the neutral andbraked neutral positions, the first of these modes being a powerspitting mode, the second a direct drive mode and the third an overdrivemode.

SUMMARY OF THE INVENTION

The present invention consists in a transmission for high-speedtrack-laying vehicles or vehicles with non-steerable wheels having anengine adapted to produce substantially constant power over a wide rangeof speeds, the transmission comprising a gearbox equipped with ahydrodynamic torque converter, said hydrodynamic torque converter havinga primary (pump) and a secondary (turbine) and functioning in a powersplitting arrangement, an output shaft of said gearbox, steering meansadapted to procure continuous variation of the turning radius, and areversing gearing associated with epicyclic output gears, and furthercomprising a positive clutch coupling the secondary of said hydrodynamictorque converter to said gearbox output shaft, together with twofreewheel clutches driving the said steering means, one of saidfreewheel clutches being driven by the vehicle engine whereas the otherone is driven by the primary of said hydrodynamic converter, whereby thehighest drive speed is selected at all times.

The gearbox preferably includes an epicycllc input gear at the input tothe hydrodynamic torque converter with a ratio of approximately 2:1.This favors operation of the drive engine at constant power over theentire range of vehicle speeds (by comparison, the ratio of thegearboxes of conventional hydrodynamic torque converter transmissionsbetween two consecutive gears is in the order of 1.6:1).

Specifically, the epicyclic input gear includes two families of planetpinions meshing with each other and one of which meshes with the annulusgear of the epicyclic input gear, which is adapted to be driven by thevehicle engine, and the other of which meshes with the sun gear of theepicylcic input gear, which is coupled to the gearbox output shaft, theplanet carrier of the epicyclic input gear driving the primary of thehydrodynamic torque converter.

It is particularly advantageous to provide a clutch and a brakeassociated with the hydrodynamic torque converter, the primary of whichcan be stopped by the brake and/or coupled by the clutch to thesecondary of the hydrodynamic torque converter.

In one particularly advantageous embodiment the transmission furthercomprises an actuator device associated with the positive clutch bywhich the positive clutch is always engaged except in a purelymechanical overdrive operating mode in which the brake is applied andthe clutch is released. A structure of this kind is simple and ruggedand, using modern synchronism detecting devices, makes it possible todispense with a clutch (costly) or an additional freewheel (one-way)clutch (eliminating the engine brake).

It is advantageous for the steering means to incorporate a variablespeed drive. Specifically, the variable speed drive comprises ahydrostatic transmission including a variable delivery pump having aninput shaft driven by the two freewheel clutches, which are mounted onthe input shaft, and a fixed capacity hydraulic motor, the transmissionfurther comprising a steering shaft through which the hydraulic motordrives the epicyclic output gears. A solution of this kind makes itpossible to avoid adding excessive weight to the transmission (anelectrical system would be heavier for equal power than theaforementioned plus pump hydraulic motor system).

The reversing gearing preferably comprises a conical triad connected tothe gearbox output shaft and to the epicyclic output gears, whichresults in a compact and rugged structure; specifically, thetransmission further comprises forward and reverse positive clutchesassociated with the reversing gearing and by which the reversing gearingis coupled to the epicyclic output gear.

It is particularly advantageous for the transmission to comprise a driveclutch having a primary adapted to be driven by the vehicle engine and asecondary driving the annulus gear of the epicyclic input gear, meansbeing provided to release the drive clutch in the neutral or the brakedneutral or the direction selection position independently of the stateof the brake or clutch associated with the hydrodynamic torque converteror of the state of the positive clutches respectively coupling thegearbox to the reversing gearing and the reversing gearing to theepicyclic output gears.

Other characteristics and advantages of the invention will emerge moreclearly from the following description and the appended drawingsrelating to one specific embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic showing a transmission in accordance with theinvention in which the functional subsystems are indicated withinchain-dotted rectangles.

FIG. 2 reproduces part of FIG. 1 showing the subsystem corresponding tothe gearbox of the transmission.

FIG. 3 reproduces another part of FIG. 1 showing the subsystem IIcorresponding to the steering means.

FIG. 4 reproduces a further part of FIG. 1 showing the subsystem IIIcorresponding to the reversing gearing and the associated subsystems IVand IV' corresponding to the output epicyclic gears.

DESCRIPTION OF THE PREFERRED EMBODIMENT

To make the description clearer and more logical, the transmission 1shown in FIG. 1 has been broken down into different functionalsubsystems I, II, III, IV and IV' which will be described in successionwith reference to the corresponding FIG. 2, 3 or 4, from the input shaft2 of the transmission connected to the drive motor 3 of the vehicle tothe output shafts 4, 4' connected to the sprocket wheels driving thetracks of said vehicle.

It should be noted that the various component parts of the transmissionare individually well known to those skilled in the art, and for thisreason they will be described only briefly.

As emerges from the figures, the gearbox of the transmission 1 is of aparticularly simple design, which results partly from the fact that thedrive engine 3 is an engine supplying essentially constant power over awide range of speeds, for example in the ratio of 1:2.

The arrangement of this transmission makes it possible to reduce thenumber of gear ratios and to improve the performance of the vehicle. Ifthe ratio of the gearbox between two consecutive gears is around 2:1 thedrive engine 3 operates at constant power over all the range of speedsof the vehicle, and in particular up to speeds as high as 80 kph.

The subsystem I includes a hydrodynamic torque converter 5 for startingthe vehicle and providing the necessary multiplication of the torque atlow speeds. The reader is reminded that the type of vehicle that theinventive transmission concerns implies a transmission spread (the ratiobetween the possible traction force on starting and that at maximumspeed) with a value of around 10.

The hydrodynamic converter 5 shown here is of known design but the wayit is used differs substantially from that of existing transmissions, aswill emerge later.

The schematic representation of the hydrodynamic torque converter 5shows a primary 6 (pump), a set of reaction blades 7 and a secondary 8(turbine), the output torque being the sum of the input and reactiontorques.

In the first mode the aim is to use the known principle of powersplitting at the input to the hydrodynamic converter 5, combined withthe fact that the drive motor, at constant power, must turn atsubstantially half its speed (but already at maximum power). It istherefore a question of multiplying by a factor of approximately 2 thespeed of the converter 5, which must therefore turn at its nominalspeed. For this purpose differential type means are employed with ratioof 2:1. A conical differential could be used, but it is simpler toprovide an epicyclic gear with two families of planet pinions meshingtogether, one family meshing with the annulus gear connected to thedrive engine and the other family with the sun gear coupled to thegearbox output shaft.

There is shown here a transmission 1 incorporating an epicyclic (orplanetary) gear 9 of the aforementioned type. The component parts of theepicyclic gear 9 are not given reference numbers in FIG. 1 in order toclarify the diagram, except for the output shaft 10 driven by saidepicyclic gear. It is therefore necessary to refer to FIG. 2 which showsthe planet pinions 11 of a first family meshing with the annulus gear 12which is coupled to the drive engine (through the intermediary of adrive clutch 13 as will emerge later); FIG. 2 also shows planet pinions14 of a second family meshing with the sun gear 15 which is coupled tothe output shaft 10. The two families 11, 14 of planet pinions meshingtogether are located on a planet carrier 16 which is driven via acoupling to the primary 6 of the hydrodynamic torque converter 5.

There is further provided a brake 17 for the power splitting function,for example a jaw or multiple-disk type brake, capable of stopping theprimary 6 of the hydrodynamic torque converter, together with a clutch18, a disk clutch, for example, capable of coupling the primary 6 to theturbine constituting the secondary 8 of said converter. It should benoted that the aforementioned brake 17 and clutch 18 can be operatedseparately, so that the primary 6 of the converter can be stopped by theassociated brake 17 and/or coupled to the secondary 8 by the associatedclutch 18. The same goes for the drive clutch 13, the primary 19 ofwhich is driven by the input shaft 2 and the secondary 20 of whichdrives the annulus gear 12 of the epicyclic gear 9. This will emergeeven more clearly from the explanations given later with reference tothe functioning of the transmission.

One essential feature of the inventive transmission is that thesecondary 8 of the hydrodynamic torque converter 5 is coupled to theoutput shaft 10 through the intermediary of a positive clutch; FIGS. 1and 2 show the coupling between the output shaft 10 and a coaxial shaft21 coupled to the secondary 8 by a positive clutch 22.

The positive clutch 22 will be associated with any known actuator device(mechanical, hydraulic or pneumatic) so that it is always engaged exceptin the third mode of operation, which is purely mechanical,corresponding to an overdrive mode in which the brake 17 is applied butthe clutch 18 is released, as will be explained in more detail inrelation to the general functioning of the transmission.

A positive clutch coupling of this kind is particularly advantageous asit makes it possible to simplify the overall design by dispensing withmembers such as a clutch (costly) or a freewheel (one-way) clutch(eliminating the engine brake) and simply by making use of modern, veryefficient synchronism detecting devices; it is further possible toprovide an engine brake that is very useful in the first and secondoperating modes (see below for explanations regarding the variousoperating modes).

In this way it is possible to use only two sliding members (the brake 17and the clutch 18 associated with the hydrodynamic converter 5), whichis highly advantageous with regard to the simplicity and reliability ofthe system.

Another fundamental feature of the invention is the drive coupling tothe steering means procuring continuous variation of the turning radius.

It should be noted that the steering means represented by the subsystemII of FIGS. 1 and 3 incorporate in this instance a variable speed drivewhich is implemented in the form of a classic hydrostatic transmissionmade up of a variable delivery pump and a fixed capacity hydraulicmotor: this is merely a preferred example, however, it being understoodthat it will be possible to use an electrical system with a generatorand a motor (although in the current state of the art this would resultin a much heavier structure).

The novelty of the inventive transmission resides in the arrangement ofthe coupling between the subsystems I and II, as described hereinafter.

A gear 23 on the input shaft 2 cooperates with a gear 24 mounted throughthe intermediary of a first freewheel clutch 25 on a shaft 26 drivingthe variable delivery pump of the steering means (pump 30): this enablesthe steering means to be driven via a first freewheel clutch 25 which isdriven by the engine 3. A gear 27 fastened to the primary 6 of thehydrodynamic torque converter 5 cooperates with a gear 28 also mounted,through the intermediary of a second freewheel clutch 29, on the shaft26 driving the pump 30: the latter therefore enables the steering meansto be driven by a second freewheel clutch 29 which is driven by theprimary 6 of the hydrodynamic torque converter, which primary is coupledto the planet carrier of the epicyclic input gear 5.

An arrangement of this kind therefore makes it possible to choose at alltimes the highest drive speed for the steering means.

The combination of the aforementioned essential features (association ofan engine offering essentially constant power over the speed range,positive clutch coupling of the converter secondary to the output of thegearbox and driving of the steering means through two freewheel clutches(one of which is driven by the engine and the other of which is drivenby the primary of the converter) makes it possible to produce atransmission perfectly suited to all-terrain track-laying vehicles andwhich is simple and rugged.

The steering means constituted by the subsystem II further comprise, inthe conventional way, a hydraulic circuit 32 connecting the variabledelivery pump 30 to the fixed capacity hydraulic motor 31. The hydraulicmotor 31 drives a steering shaft 33 operating on the sun gears of theoutput reduction gears (subsystems IV and IV') through the intermediaryof associated gears such as the gears 34 and 35.

As can be seen in FIGS. 1 and 4, the transmission is completed byreversing gearing (subsystem III) and by two epicyclic output gears(subsystems IV and IV'), in an entirely conventional way.

The reversing gearing is in the form of a conical triad (gear 36fastened to the output shaft 10 and associated gears 37, 38). Othersolutions could be used, however, for example a conical systemassociated with reversing gearing having five straight gears; thevariant shown has the advantage of simplicity, however.

The reversing gearing communicates motion to an associated transverseshaft 41 by any appropriate means: here there is shown a forwardpositive clutch 39 and a reverse positive clutch 40, but strictlyspeaking progressive clutches could be used in place of these positiveclutches.

The shaft 41 is coupled to the two epicyclic output gears in theconventional way.

The gear represented by the subsystem IV comprises, as shown in FIG. 4,an annulus gear 42 coupled to the shaft 41, a family of planet pinions43 on a planet carrier 44 coupled to the output shaft 44. The sun gear45 of the output gear carries a gear 46 coupled to the steering shaft 33through the intermediary of a gear 47. The gear represented by thesubsystem IV' is identical (except for the gear 47), which is why itscomponent parts have been given the same reference numbers but with aprime.

Before describing the functioning of the inventive transmission in moredetail, with its various modes and the changes between them, it isnecessary to point out that the drive clutch 13, the primary of which isdriven by the input shaft 2 and the secondary of which drives theannulus gear of the epicyclic gear of the gearbox, may be released inthe neutral position (braked or otherwise) or in the direction selectionposition independently of the operation of the brake 17 and/or theclutch 18 associated with the primary of the hydrodynamic converter 5and the operation of the positive clutch 22 coupling the gearbox to thereversing gearing and of the positive clutches 39, 40 coupling thereversing gearing to the epicyclic output gear.

Consider now the various possible situations.

Neutral

In the neutral position the drive clutch 13 is released as are theclutch 18 and the brake 17 associated with the hydrodynamic converter 5;the positive clutches 39, 40 are open but the positive clutch 22 isengaged.

Thus with the exception of the positive clutch 22 all the componentparts of the transmission are open in the neutral position.

Braked Neutral

This situation is specific to track-laying vehicles such as tanks: itenables the tank to turn on itself, pivoting about a vertical medianaxis.

In this position all the elements are engaged, except of course for thedrive clutch 13, in order to immobilize totally the shaft 41: the clutch18 and the brake 17 associated with the hydrodynamic converter 5 areengaged, the positive clutch 22 remains engaged and one or other of thedirection positive clutches 39 or 40 is engaged. The vehicle then turnsreadily on itself using the steering means driven by the shaft 2(without the gearbox halted by the brake 17 being driven, since thedrive clutch 13 is open).

The maneuver is easy and flexible, even on a slope.

Reversing or Selecting Direction (From Neutral)

The component parts of the transmission are in the same position aspreviously.

With the vehicle stopped, the "braked neutral" maneuver is initiatedrapidly and one of the positive clutches 39 or 40 is therefore engaged,as appropriate to the required direction, the other positive clutchbeing released if it was previously engaged.

The same applies in the case of reversing the direction.

First Mode

Once the direction is chosen the first mode is entered from the neutralposition.

In this situation the drive clutch 13 is operated; the positive clutch22 remains engaged while the clutch 18 and the brake 17 associated withthe hydrodynamic converter 5 are released. The positive clutch 39 or 40corresponding to the direction already chosen naturally remains engaged.

By virtue of the converter the gearbox then functions as a hydrodynamicpower splitter, which enables the vehicle to start and provides thenecessary multiplication of the torque at low speeds. With the powersplit the motor can run at a speed between half its maximum speed (forexample, 1 200 rpm) and a speed approaching the maximum speed anddepending essentially on the characteristics of the converter (2 200 to2 300 rpm approximately).

Thus in the first mode when the vehicle is started the converter isdriven with the multiplication ratio (of nearly 2:1) provided by theepicyclic input gear, the sun gear of which starts with the vehicle; theengine is then loaded by the converter and is initially held athalf-speed.

As the vehicle accelerates the engine can turn faster and faster whilethe primary of the converter turns at substantially the same speed (forexample, 2 600 to 3 000 rpm).

As the steering means are driven by freewheel clutches they are in thefirst mode driven by the pair of gears 27, 28 and the freewheel clutch29 as these are naturally the components turning the fastest.

If required, the direct drive mode of the converter may be selected toselect drive mode of the converter may be selected to select the secondmode in order to reduce the engine speed to approximately half-speed.

Second Mode

The second operating mode corresponds to direct drive, the hydrodynamicconverter no longer contributing to the transmission of power.

In this situation the drive clutch 13 is naturally still operating andthe positive clutch 22 and the positive clutch 39 or 40 (as appropriate)remain engaged; the difference as compared with the previous moderesides in the operation of the clutch 18 associated with the converter,which "bridges" said converter (the brake 17 then remains released).

This mode of operation is purely mechanical, as the hydrodynamicconverter 5 rotates with the engine.

In this case the steering means are driven by the two freewheel clutches25 and 29.

At the end of the second mode the engine will have again reached itsmaximum speed.

For changing up the change from the first to the second mode isperformed by operating only the clutch 18, which cancels out the slip ofthe converter, and reduces the engine speed to half the maximum speed.The changeover point will be chosen as appropriate to thecharacteristics of the converter, so that the engine is actually athalf-speed.

Conversely, on changing down, the changeover from the second mode to thefirst mode is easily achieved by simply releasing the clutch 18 (thechange from the first mode to the second mode and vice versa is of the"powershift" type, that is to say it does not interrupt the tractionforce).

Third Mode

The third operating mode corresponds to a purely mechanical overdrivemode exploiting the fact that the epicyclic input gear 9 has a ratio inthe vicinity of 2:1. The engine can run up to speed again and drive thevehicle at its maximum speed.

In this situation the drive clutch 13 is still operated but the clutch18 associated with the converter is released and the brake 17 isapplied. The positive clutch 39 or 40 remains engaged but the positiveclutch 22 is open.

The annulus gear of the epicyclic input gear 9 is stopped by the brake17 so that it is possible to stop the input to the hydrodynamic torqueconverter 5 the secondary of which is decoupled from the output of thetransmission because the positive clutch 22 is open.

In this case the steering means are driven by the pair of gears 23, 24and the freewheel clutch 25, as these are turning the fastest.

The engine, the speed of which has again dropped to half-speed onchanging from the second to the third mode, can in this overdrive modedouble the speed of the vehicle by returning to its maximum speed.

On changing up the changeover from the second to the third mode iseffected by releasing the clutch 18 and applying the brake 17, theconsequence of which is to establish, through the epicyclic input gear9, a ratio of approximately 2:1 between the drive shaft 2 and the outputshaft 10: this therefore reduces the engine speed to half-speed. At thesame time the positive clutch 22 is naturally open to decouple theoutput of the hydrodynamic torque converter 5 from the output shaft 10(if this positive clutch were not released the vehicle would be braked).

Conversely, to change down from the third mode to the second mode, afterreleasing the brake 17 and engaging the clutch 18 the engine isaccelerated to its maximum speed to bring about synchronization and thepositive clutch 22 is then re-engaged. In practise synchronization canbe controlled by a microprocessor-based computer controlling automaticmode changing.

The invention is not limited to the embodiments that have just beendescribed but to the contrary encompasses any variant thereof usingequivalent means to achieve the essential characteristics as specifiedin the claims. Specifically, there have been described and shown onlythe essential active components of the inventive transmission althoughit goes without saying that for reasons of architecture or technology itwould be possible to provide one or more intermediate gears betweenthese various components.

I claim:
 1. Transmission for high-speed track-laying vehicles orvehicles with non-steerable wheels having an engine adapted to producesubstantially constant power over a wide range of speeds, saidtransmission comprising a gearbox equipped with a hydrodynamic torqueconverter, said hydrodynamic torque converter having a primary (pump)and a secondary (turbine) and functioning in a power splittingarrangement, an output shaft of said gearbox, steering means adapted toprocure continuous variation of the turning radius, and a reversinggearing associated with epicyclic output gears, and further comprising apositive clutch coupling the secondary of said hydrodynamic torqueconverter to said gearbox output shaft, together with two freewheelclutches driving the said steering means, one of said freewheel clutchesbeing driven by the vehicle engine whereas the other one is driven bythe primary of said hydrodynamic converter, whereby the highest drivespeed is selected at all times.
 2. Transmission according to claim 1wherein said gearbox includes an epicyclic input gear at the input tosaid hydrodynamic torque converter with a ratio of approximately 2:1. 3.Transmission according to claim 2 wherein said epicyclic input gearincludes two families of planet pinions meshing with each other and oneof which meshes with the annulus gear of said epicyclic input gear,which is adapted to be driven by the vehicle engine, and the other ofwhich meshes with the sun gear of said epicyclic input gear, which iscoupled to said gearbox output shaft, the planet carrier of saidepicyclic input gear driving the primary of said hydrodynamic torqueconverter.
 4. Transmission according to claim 1 comprising a clutch anda brake associated with said hydrodynamic torque converter, the primaryof which can be stopped by said brake and/or coupled by said clutch tothe secondary of said hydrodynamic torque converter.
 5. Transmissionaccording to claim 4 further comprising an actuator device associatedwith said positive clutch whereby said positive clutch is always engagedexcept in a purely mechanical overdrive operating mode in which saidbrake is applied and said clutch is released.
 6. Transmission accordingto claim 1 wherein said steering means incorporate a variable speeddrive.
 7. Transmission according to claim 6 wherein said variable speeddrive comprises a hydrostatic transmission including a variable deliverypump having an input shaft driven by said two freewheel clutches, whichare mounted on said input shaft, and a fixed capacity hydraulic motor,and further comprising a steering shaft through which said hydraulicmotor drives said epicyclic output gears.
 8. Transmission according toclaim 1 wherein said reversing gearing comprises a conical triadconnected to said gearbox output shaft and to said epicyclic outputgears.
 9. Transmission according to claim 8 further comprising forwardand reverse positive clutches associated with said reversing gearing andby which said reversing gearing is coupled to said epicyclic outputgears.
 10. Transmission according to claim 9 comprising a clutch and abrake associated with said hydrodynamic torque converter, the primary ofwhich can be stopped by said brake and/or coupled by said clutch to thesecondary of said hydrodynamic torque converter, and further comprisinga drive clutch having a primary adapted to be driven by the vehicleengine and a secondary driving the annulus gear of said epicyclic inputgear, means being provided to release said drive clutch in the neutralor the braked neutral or the direction selection position independentlyof the state of said brake or clutch associated with said hydrodynamictorque converter or of the state of said positive clutches respectivelycoupling said gearbox to said reversing gearing and said reversinggearing to said epicyclic output gears.